Amazon hints at one of its best-kept secrets: How many Prime members it has

Amazon has historically kept its number of Prime subscribers close to the vest, but in its latest 10-K filing, the online retailer hinted at just how many members it might have.

Still, there are some discrepancies.

In its filing with the Securities and Exchange Commission last week, Amazon added a new line item to its annual report: retail subscription services.

The company said it generated $6.4 billion in revenue related to that item last year.

Guggenheim Securities analyst Robert Drbul estimates that shakes out to about 65 million Prime members. Cowen & Co. analyst John Blackledge calculated an even higher number of subscribers. His estimate pegs the Prime subscriber base at closer to 80 million globally.

An Amazon spokeswoman declined to offer additional details.

The precise number of Prime members is difficult to nail down given the different subscription options Amazon offers. In the U.S., Amazon charges $99 for an annual Prime membership, which gives subscribers free two-day shipping and access to other perks like its streaming services. Amazon also offers a monthly membership for $10.99 and Prime Video membership for $8.99 a month.

The latest estimates from Consumer Intelligence Research partners assumes about 19 percent of Prime members have opted for a monthly payment plan.

Prime has been successful not only in generating subscription renewals but getting shoppers to spend more with Amazon to get the most out of their perks.